The present invention relates to a numerical control system which performs turning operations by moving a cutter or a work in synchronism with the rotation of a spindle, and more particularly to a numerical control system for turning operations such as tapping, polygonal machining, cam turning and so on.
In the prior art numerical control apparatus which moves a cutter or a work in synchronism with the rotation of a spindle, position command values to be given to a axis which moves the work or the cutter are arithmetically calculated and outputted in correspondence with a detected rotational angle of the spindle without changing the phase position of the marker pulses from a spindle pulse generator (hereinafter referred to as "a spindle PG") against A- and B-phase pulses. More specifically, referring to FIG. 1, the spindle PG 8 is fixed on a spindle for detecting the rotational angle thereof to output various signals in the form of pulses comprising generally A- and B-phases and the marker phase. Since the A- and B-phases are defined in respect of the phase relationship, the rotational direction may generally be determined simply by detecting the phase relationship between the two phases. The signals in the marker phase are the signals which enable detection of the rotational angle of 360 degree or less by outputting one pulse for one rotation.
A numerical control system 100 is adapted to output the rotational angle value of the spindle from a rotational angle detector 31, and to output axis command values in correspondence with the detected values from a interpolator 32. A case where a starting point for tapping is to be changed is assumed; since the rotational angle value is the value counted from the rotational angle of the marker pulse, if the point is to be moved by 10 degrees from the value or the reference, then value of the angle is to be subtracted from the above rotational angle value after having matched the units of the both apparatus. If the difference is positive or zero, the positional command value of the axis in correspondence with the subtracted value is to be arithmetically calculated and then outputted.
Such operation has been generally performed in the numerical control causing no significant problems as functions of positional command values can be generated only in the period of about 1-10 msec, and the positional loop gain of the feed axis remains at about 20-80 sec.sup.-1 at the current technical level. However, the demand for an extremely specialized turning operation at a high speed and at a high precision has increased in recent years. Therefore, for such a highly specialized purpose, it is necessary to use a numerical control system which is exclusively adapted for that purpose with specialized functions.
Such a specialized numerical control system has been proposed, for example, by Japanese Patent Application No. 190173/1988 entitled "Numerical control system with a detection function of follow-up errors". In that system, control is performed as follows when the rotational angle of the spindle of a numerically controlled machine is synchronously controlled with the position of the feed axis. More specifically, the apparatus first reads out a positional command value for the feed axis corresponding to the actual rotational degree detection value out of a data table which is stored in advance in correspondence with the rotational angle of the spindle. The actual positional detection value of the feed axis is subtracted from the read-out command value to obtain the positional deviation of the feed axis. Then, a correction value corresponding to the actual rotation angle detection value of the spindle is read out from the data table of the correction values of the feed axis which is stored in advance in correspondence with the rotational angle of the spindle, and the read-out value is added to the positional deviation. By synchronously controlling the apparatus based on thus obtained value, a special turning operation becomes possible.
In the above mentioned numerical control system, since the data tables have been prepared in advance, and the apparatus does not require interpolation during the numerical control, a period as short as about 0.1 msec can be realized. The higher stiffness of the feed axis achieved in recent years has improved the positional loop gain to approximately 100-300 sec.sup.-1. For instance, when the rotational speed of the spindle is 3000 RPM, and a work is to be cam-turning in an elliptical shape, the period of one rotation of the spindle is 20 msec, and there are 4 positional command values of the numerical control system for one rotation of the spindle if the interpolation period is 5 msec. A precisely elliptical shape, therefore, cannot be machined. However, the new numerical control system having the aforementioned special turning operation can issue 200 commands because the period is at least 0.1 msec.
The above mentioned numerical control system is still not quite satisfactory because it is too time consuming as the data tables must be re-prepared when the rotational angle position of a work with respect to the spindle chuck is to be changed for synchronized control of the rotational angle of the spindle and the position of the feed axis. Especially, it consumes too much time because one needs to re-prepare all the data tables corresponding to plural rotations instead of the data table for one rotation of the spindle. When a work has a shape equivalent to one rotation of the spindle and when the starting point for the designated shape has to be changed continuously by the position parallel to the direction of the rotational central axis of the spindle; in other words, when a work shaped as shown in FIG. 6 is to be lathed, the data tables should be re-prepared within one rotation of the spindle or functions should be generated in real time while the work is being machined. In short, it was quite difficult to increase the number of spindle rotations.
As shown in FIGS. 2A-2B when a work should be continuously shaped into at least two types of forms having the same phase angles but different amplitudes, and if the rotational angle position of the work in respect of the spindle chuck is varied by simply shifting the data table thereof, a considerable gap is inevitably generated in the positional command value which shifts the data table with the marker pulses generated from the spindle. The gap poses a serious problem in control because it requires a higher speed and acceleration of a feed axis.